The present invention relates generally to devices for muffling noises emitted from the combustion air induction system of an internal combustion engine.
Two-cycle internal combustion engines have seen increasing use in the recreational vehicle market. The aspiration system of a two-cycle engine causes air to be gulped into the carburetor system and results in a loud noise having a frequency which is a function of the engine speed. These factors have created a great deal of interest in silencing two-cycle engine air induction systems.
Early air intake silencers merely consisted of enclosures, such as a console or hood over the engine or a box around the carburetor system, lined with sound absorbing material. While these constructions worked very well at frequencies above 1500 Hertz, they were marginally effective on low frequency intake noise and often were constructed such that the sound absorption material became saturated with fuel from carburetor spit-back.
As two-cycle engines became more powerful and thus, noisier, it became necessary to make silencers which would muffle intake noise in the frequency range below 1500 Hertz. Heretofore, this has been done by constructing air intake silencers according to the principles applied to exhaust mufflers. The simplest of these exhaust muffler designs is a tail pipe attenuator, a low pass acoustical filter which consists of an expansion chamber having a length of pipe connected thereto. Studies show that this type of silencer will attenuate noise only in certain limited frequency ranges that are a function of the tail pipe length and that other frequencies may actually be amplified. The studies also show that the maximum degree of attenuation is determined by the volume of the expansion chamber, the larger the chamber the greater the attenuation. Because the intake noise harmonics shift in frequency as the engine speed changes, it is impossible to design a simple silencer, using tail pipe attenuator principles, with a broad enough attenuation range to cover a substantial part of the frequencies under 1500 Hertz. Accordingly, the most effective silencers heretofore constructed have included several reactive chambers tuned for different frequency ranges and many of these silencers have had one or more of the disadvantages of being large, complex, expensive and restrictive to air intake inflow which adversely affects the carburetor process.